1. Field of the Invention
The present invention relates to the hydrolysis of lower alkyl dicarboxylates, the alkyl moieties of which having from 1 to 4 carbon atoms, into dicarboxylic acids.
2. Description of the Prior Art
Very generally in this art, the hydrolysis of esters is carried out in homogeneous medium either using an acid or using a base.
The use of a base leads to the formation of metal salts with the obvious disadvantages thereof.
The acidic hydrolysis of adipates presents the problem of the two-phase nature of the medium, due to the very low miscibility of water and the adipate. This is the reason it has been recommended to use mixtures of solvents (see, for example, Journal of Indian Chemical Society, 48(9), pages 811-2), which markedly complicates the subsequent separation and purification of the adipic acid obtained. Additionally, the use of a liquid acid presents significant problems of corrosion of the apparatus.
It has also been proposed to carry out the hydrolysis of adipates in heterogeneous medium on acidic resins.
Thus, an article published in Synthetic Communications, 19, pages 627-631 (1989) describes the hydrolysis of dimethyl adipate in an aqueous suspension of Dowex-50 resin at reflux. This type of suspension reaction does not solve the problem of displacement of the equilibrium of the reaction by removal of the methanol formed.
EP-A-0,056,489 proposes a solution to this problem, and describes a continuous process of hydrolysis, by water, of alkyl adipates containing 1 to 4 carbon atoms, at high temperature and in the presence of strongly acidic ion-exchange resin. The principal characteristics of this process is withdrawing part of the water/adipate reaction mixture on several plates of the column, below the region of delivery of this mixture into the column, the part thus removed is then passed onto a strongly acidic ion exchanger and is then recycled. Adipic acid is thus obtained at the base of the column, whereas alcohol/water mixtures are removed at the column head.
This '489 process requires the addition of a number of circuits to the column to ensure that the hydrolysis reaction proper is carried out, and its implementation is thus complex.